Systems are well known in the art for delivering materials such as bone cement to a target site for medical treatment. One particular use of these types of systems is to treat compression fractures caused by trauma, metastasis, or osteoporosis. A compression fracture occurs when a normal vertebral body of a spine has collapsed or compressed from its original anatomical size. Typically, these vertebrae fail at an anterior cortical wall causing a wedge shaped collapse of the vertebra. Fractures can be painful for the patient typically causing a reduced quality of life. Treatments to repair these fractures are performed to reinforce the fractured bone, alleviate associated pain, and to prevent further vertebral collapse.
One method of treating compression fractures, called balloon assisted vertebroplasty, typically uses fluoroscopy to establish a percutaneous passage in the bone or vertebral body to be treated. This is followed by the insertion of an inflatable balloon-like device into the passage in the vertebral body. Liquids, typically called contrast media, are used to inflate the balloon-like device to compact the cancellous bone about the balloon and/or bone marrow toward the inner cortical wall of the vertebral body, thereby resulting in an enlargement of the passage creating a cavity. The balloon-like device is then deflated and removed from the vertebral cavity, leaving behind a cavity. A biocompatible filling material, such as polymethylmethacrylate (PMMA) bone cement is then delivered while in its flowable form into the cavity. This delivery is performed by using pressure type devices. The filling material is then allowed to set to a hardened condition to provide internal structural support to the bone.
Balloon-like devices require exertion of pressure for expansion of the balloon and/or insertion of flowable materials into the balloon. These balloon-like devices can require high inflation pressures, sometimes as high as 400 psi., in order to obtain the desired cavity size or compaction. These balloon-like devices have been known to fail during inflation due to the high inflation pressures, thin balloon membranes required to fit into the percutaneous passage, and sharp tools or bony structures piercing the membranes. Other mechanical devices have been suggested in order to tamp the bone and create a cavity for subsequent filling with bone cement. In today's art, filling the cavity created by a balloon or tamping device requires applying a pressure to the flowable material. Syringe like devices are typically used to create the pressure to flow the material from a chamber and down a channel into the bone. Once the flowable materials leave the delivery system, they flow toward lower pressure regions through the path of least resistance until the pressure has neutralized with its surroundings. This action occurs in an uncontrollable manner where the user cannot influence the flowable material. In other words, flow of the material and the path that the material takes outside of the delivery system cannot be influenced by the practitioner. These flowable materials have been know to flow along fracture lines, into vascular structure as well as into other cracks, holes or spaces in the bone that may or may not have been known to the practitioner.
Another procedure that relies on delivering bone cement under pressure to treat compression fractures is called vertebroplasty. This method of stabilizing bone follows very much the balloon assisted vertebroplasty procedure described above, except vertebroplasty does not utilize balloons or tools to create a cavity prior to the injection of bone cement. Vertebroplasty is typically performed under fluoroscopic guidance and includes the placement of a cannula into the vertebral body to provide a pathway for the bone cement to enter the vertebral body.
During vertebroplasty, low bone cement viscosity and high injection pressures tend to disperse the bone cement throughout the vertebral body. By utilizing injection pressure, the bone cement takes the path of least resistance, which in some instances can lead to undesirable leaking or extravasations outside of the vertebral body.
It is known in the medical community that instances of leaking outside of the vertebral body occur with the above described procedures. For the most part, these leaks have not caused severe symptoms or complications requiring additional medical intervention. Nevertheless, the following complications have been associated with leaks outside of the vertebral body: epidural hematoma; intrusion into the spinal canal with permanent paralysis, radiculopathy, paresthesias or loss of motor function; pulmonary embolism; pneumothorax; and death.
Another limitation of the current pressure delivery system is the difficulty of visualizing the flowable materials using a fluoroscope. Fluoroscopes are traditionally used by the medical practitioner in order to identify the bony structure, the radiopaque instruments used and the radiopaque flowable materials injected as described above. As mentioned earlier, the practitioner cannot influence the flow of the materials. Once the materials have left the delivery system, these materials can flow through thin cracks or small crevices in a manner where the practitioner cannot see the image of this thin flow on the fluoroscope. As one can appreciate, the inability to see thin flow fronts can mislead the practitioner into applying more pressure to deliver more flowable materials, even when the thin flow fronts are leaking outside the vertebral body and into undesirable locations. An example of a filling material for use in vertebroplasty to overcome these problems can be found in U.S. Pat. No. 6,231,615 to Preissman. Preissman discloses an enhanced visibility composition of a flowable material with radiopaque particles up to 350 (micron) and tracer elements having a size between 570 (micron) and 2200 (micron) for improving the visualization with medical imaging. Preissman, however, did not consider the problem when thin flow fronts exist and the disclosed tracers are separated from the flow when the bony structure restrains the tracers, effectively filtering them, as the flow continues down thin sections.
Recently, in an attempt to overcome these problems, systems have been developed to treat compression fractures by delivering structural elements to distract tissue surfaces forming the collapsed vertebral body. A shortcoming of these systems is the lack of complete stabilization of the bony structure and the lack of a permanent fixation of the implant to the bone. It is believed that motion of a bony structure of cancellous bone within the vertebral body may result in pain to the patient. Thus, it is desirable to stabilize the cancellous bone to prevent this motion.
U.S. Pat. No. 6,595,998 to Johnson et al. discloses a tissue distraction device for treating compression fractures by inserting a plurality of wafers into a vertebral body to form a wafer stack. Once the wafer stack is formed, the bone cement can be delivered into the vertebral body around the wafer stack to lock the wafers together and form a stable implant. The wafer stack provides support on upper and lower sides of the vertebral body, but may not provide uniform support on all sides. Also, Johnson et al. does not disclose how much bone cement is delivered and/or whether enough is delivered to stabilize the bony structure of cancellous bone within the vertebral body. Furthermore, this delivery occurs through relatively little control of the flow of pressurized bone cement during delivery, much like as described above.
Another prior art system is described in U.S. Patent Application Publication No. 2005/0278023 to Zwirkoski. In this system, a plurality of segments, flexibly connected to one another, are inserted into a vertebral body to treat a compression fracture. The system includes an applicator having a rotary driver, such as an auger or a cog wheel, for transporting the plurality of flexibly connected segments through a cannula and into the vertebral body. Zwirkoski suggests passage of fluent materials such as bone cement into the vertebral body concurrent with the segments. However, Zwirkoski fails to disclose how to perform this concurrent delivery. Moreover, Zwirkoski does not disclose how much bone cement is delivered and/or whether enough is delivered to stabilize the bony structure of cancellous bone within the vertebral body.
Thus, there is a need in the art for a system that is capable of simultaneously delivering structural elements and fluent material, e.g., delivering a mixture of elements and fluent material, to a vertebral body for medical treatment such that the implant materials are delivered in a controlled manner with a low fluent pressure to reduce leaking There is also a need for a system that is capable of adjusting relative amounts of elements and fluent material in the mixture delivered to customize a particular procedure based on a patient's anatomy and structural requirements of the final implant to suitably stabilize the vertebral body. There is also a need to improve the visualization during implantation by increasing the effective radio-opacity of the implant by preventing thin flow fronts.